1. Technical Field
The present invention generally relates to systems used to measure forces in pneumatic tires. More particularly, the present invention relates to a system for measuring tire force generation and tire force transmission as a loaded tire rolls over a road surface. Specifically, the present invention relates to a system and method for measuring tire forces wherein the force contributions of the test machine are separately measured from and added into the resulting force measurement.
2. Background Information
Tire manufacturers desire to compare the forces generated and transmitted by different tire constructions to determine a preferred tire construction for a particular application. The subject forces are created when a loaded tire rolls over a road surface. A tire rolling over a road surface creates vibration forces that will create tire noise. Minimizing or controlling tire noise is one of the objects of tire design. Force testing provides the tire designer a method for comparing alternative tire constructions.
Tire forces have been measured in the past by mounting the subject tire on a rim that is then mounted to a rotor. The rotor is rotatably mounted on a bearing housing that is connected to a load cell or force measurement device. The load cell or force measurement device is configured to measure three axial forces and the moments about the reference axes. One type of load cell is known in the art as a Kistler load cell and is available from Kistler of Switzerland. The tire is brought into contact with a rotating simulated road surface that causes the tire and rotor to rotate. The load cell measures the forces and delivers the measurements to a recording device such as a computer that records the load cell forces. The readings may be displayed for use by investigators in a variety of different ways. One method of displaying the data is to perform a Fourier transformation on the data to display it in the frequency domain.
In past systems, the inertial forces experienced by the rotor, the bearing housing and the measurement device would be measured by the load cell in combination with the tire forces. The inertial forces do not influence force measurements at low frequencies because the inertial forces are generally higher frequency forces. At the higher frequencies, the machine forces combine with the tire forces to yield useless results. For instance, those skilled in the art know that tires have certain measurable resonances such as the resonances caused by the tire cavity and the body cords. Another known resonance occurs in the wheel to which the tire is mounted. Although these resonances are known to exist, the existing tire force measurement systems do not accurately reveal these resonances because the forces of the test machine hide these resonances. The persons testing the tires desire a system that accounts for the machine forces when it measures the tire forces.
In addition to the force accounting, the users desire a system that is as easy to use as prior art systems. When multiple tire constructions must be measured and compared, each tire must be loaded onto the test machine and tested. The test tires are currently loaded onto machines by using standard lug nuts to hold the wheel to the rotor. The entire loading and test process takes less than 5 minutes. Any improved system should be as quick and easy to use as these systems so that they will be readily accepted by those who perform the tests.
The invention provides a system and method for measuring tire forces wherein the force contributions of the test machine are separately measured and accounted for in the overall measurement of the tire force. The invention accounts for the machine forces by equipping the test station with a plurality of accelerometers that allow the forces and the moments of the components of the test station to be independently calculated from the overall measured forces.
The system of the invention allows wheels to be quickly mounted to the test station so that they may be tested as quickly as in known test stations. In one embodiment of the invention, a slip ring is disposed between the bearing housing and rotor so that the data from the rotor accelerometers may be gathered by the data gathering device of the test station. The slip ring allows the tire-wheel assemblies to be mounted and dismounted from the rotor without disconnecting the accelerometers from the data gathering device.